Unconventional hydrocarbon reservoirs are any reservoir that requires special recovery operations outside the conventional operating practices. Unconventional reservoirs include reservoirs such as tight-gas sands, gas and oil shales, coalbed methane, heavy oil and tar sands, and gas-hydrate deposits. These reservoirs have little to no porosity, thus the hydrocarbons may be trapped within fractures and pore spaces of the formation. Additionally, the hydrocarbons may be adsorbed onto organic material of an e.g. shale formation.
The rapid development of extracting hydrocarbons from these unconventional reservoirs can be tied to the combination of horizontal drilling and induced fracturing (called “hydraulic fracturing” or simply “fracking” or “frac'ing”) of the formations. Horizontal drilling has allowed for drilling along and within hydrocarbon reservoirs of a formation to better capture the hydrocarbons trapped within the reservoirs. Additionally, increasing the number of fractures in the formation and/or increasing the size of existing fractures through fracking may increase hydrocarbon recovery.
In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a proppant material is pumped downhole into the formation at a pressure sufficiently high enough to cause fracturing of the formation or enlargement of existing fractures in the reservoir. Proppant material remains in the fracture after the treatment is completed, where it serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the well bore through the fracture. The spacing between fractures as well as the ability to stimulate the fractures naturally present in the rock may be major factors in the success of horizontal completions in unconventional hydrocarbon reservoirs.
While there are a great many fracking techniques, one useful one is “plug-and-perf” fracking. Plug-and-perf completions are extremely flexible multistage well completion techniques for cased hole wells. Each stage can be perforated and treated optimally because options can be exercised up to the moment the perforating gun is fired. The engineer can apply knowledge from each previous stage to optimize treatment of the current stage.
The process consists of pumping a plug-and-perforating gun to a given depth. The plug is set, the zone perforated, and the tools removed from the well. A ball is pumped downhole to isolate the zones below the plug and the fracture stimulation treatment is pumped in. The ball-activated plug diverts fracture fluids through the perforations into the formation. After the stage is completed, the next plug and set of perforations are initiated, and the process is repeated moving further along the well.
Improvements in hydrocarbon recovery with fracking depend on fracture trajectories, net pressures, and spacing. Thus, the ability to monitor the geometry of the induced fractures to obtain optimal placement and stimulation is paramount. An induced fracture may be divided into three different regions (hydraulic, propped, and effective), but out of the three fracture dimensions, only the last one is relevant to a reservoir model, and may be used to forecast future production.
One common way of evaluating the geometry of hydraulic fractures during well stimulation is through microseismic measurements. However, this method has a few disadvantages. First, it is an indirect method, as microseismicity captures the shear failure of well stimulation, but not tensile opening of the hydraulic fracture itself. In addition, the physical meaning of microseismic events and how they relate to the hydraulic fracture is still widely debated in the literature. Further, the method is subject to a significant uncertainty in the location of the microseismic events.
Another common method used in industry is pressure-transient analysis or “PTA”. But, this method often leads to a wide range of potential fracture geometries.
PTA, Rate Transient Analysis or “RTA” and numerical modeling are widely used techniques to characterize effective fracture dimensions and fracture conductivity. Unfortunately, as these methods analyze the combined contribution of all induced fractures and rely on simplistic assumptions of the induced fracture system, they often lead to non-unique solutions and require additional data to further constrain the range of potential outcomes.
All of current methods used to estimate fracture dimensions and horizontal stresses can only be applied on a limited number of wells because of the significant incremental cost (procedure and additional equipment) or the time/effort required to complete the assessment.
Thus, what is needed in the art are improved methods of evaluating the hydraulic fracturing for every well being hydraulically stimulated. Although hydraulic fracturing is quite successful, even incremental improvements in technology can mean the difference between cost effective production and reserves that are uneconomical to produce.